1. Field of the Invention
The present invention relates to a diamond semiconductor device and also to a method of producing the same. More particularly, the present invention is concerned with a diamond semiconductor device having an electroluminescence device which includes a luminescent layer made of a diamond crystal.
2. Description of the Related Art
In general, a diamond exhibits a band gap of about 5.4 eV which is much greater than that of silicon, as well as a large mobility of holes. In addition, mobility of electrons is as large as that exhibited by silicon. Furthermore, a diamond has a small specific dielectric constant and large thermal conductivity. For these reasons, a diamond is considered to be very useful as a semiconductor material.
Under this circumstance, studies have been made on semiconductors using diamonds prepared by vapor phase synthesis. For instance, methods of forming semiconductor devices using diamond films are disclosed in Japanese Patent Laid-Open Publication Nos. 59-208821, 59-213126 and 60-246627. A report also has been made concerning Schottky barrier diodes which utilize a rectification effect between a metal and a p-type semiconductor diamond, as in a Extended Abstracts (The 50th Autumn Meeting, 1989); The Japan Society of applied Physics, pp 383-384, lecture Nos. 28a-E-7, 28a-E-8 and 28a-E-9.
In regard to an electroluminescence device (referred to as an "EL device", hereinunder) which employs a luminescent layer made of diamond crystal formed by vapor phase synthesis, the present inventors have proposed an EL device having double-insulation structure in JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 28, No. 10, pp L1848 to L1850 (1989). A Schottky type light emitting diode also has been proposed by Nishibayashi et al., in a Extended Abstracts (The 36th Spring Meeting, 1989); The Japan Society of Applied Physics and Related Societies, 2a-N-8, p 481, 1989.
The known semiconductor devices using diamond, however, could not provide satisfactory characteristics. For instance, the level of the operation current is undesirably limited when the devices have pn junctions, which are basic constructions in semiconductor devices. This is mainly attributed to the fact that n-type semiconductors having low resistance levels are difficult to obtain with diamond.
Japanese Patent Laid-Open Publication Nos. 59-208821, 59-213126 and 60-246627 disclose various methods for obtaining n-type semiconductors, such as ion implantation of phosphorus, arsenic or antimony into a diamond, and doping of diamond with a material gas containing a hydrogenated compound of the above-mentioned substances. Doping with such an element, however, generally results in a high resistance value of 10.sup.2 .OMEGA..cm or greater, failing to meet the demand for low resistance values (not greater than several tens of .OMEGA..cm). Thus, it has been impossible to produce a pn junction type diamond semiconductor device which can operate with a large electric current, by using an n-type semiconductor doped with phosphorus, arsenic and antimony.
A Schottky barrier diode, which has a rectification function similar to that of pn junction, undesirably has inferior dielectric strength against backward current and exhibits a large leak of backward electric current. Furthermore, this type of diode cannot be produced with a high degree of reproducibility, due to the fact that the rectification characteristic has a large dependency on the state of the interface between the metal and the diamond semiconductor.
Furthermore, production of a Schottky light emitting diodes requires an impractically high production cost, because it essentially employs homo-epitaxial growth on a diamond single crystal substrate which is formed by a high-pressure synthesis process.
The aforementioned EL device proposed by the present inventors employs a ploy crystalline diamond formed on an ordinary silicon wafer so that it can conveniently be mass-produced at a low cost. In putting this EL device to practical use, however, the following problems have been encountered in some cases.
FIG. 10 shows an EL device having a double-insulation layer structure proposed by the present inventors. This device has a diamond layer 14 which is sandwiched between insulating layers 12, 13 and 15 as illustrated. The lower insulating layers 12 and 13 under the diamond luminescent layer 14 are undesirably subjected to ions and radical seeds containing hydrogen, oxygen and carbon at a high temperatures of 800.degree. C. or higher during the formation of the diamond luminescent layer 14. Consequently, these insulating layers are damaged and suffer from degradation in their insulating effect, i.e., reduction in insulating dielectric strength, resulting in a degradation in the durability of the EL device as a product.
Furthermore, the use of poly crystalline diamond as an the luminescent layer causes problems such as the consequent impediment to movement of electrons at grain boundaries. In addition, an amorphous carbon or graphite phase in the grain boundaries allows leakage of electric current, resulting in problems such as impairment of luminescence and durability of the device.